1. Field of the Invention
The present invention relates to a catadioptric reduction optical system which is suitably applied to a projection optical system for reductive projection in a projection exposure apparatus used, for example, when a semiconductor device, a liquid crystal display device, or the like is made by a photolithography process. In particular, it relates to a catadioptric reduction optical system having a resolution of a quarter micron unit in the ultraviolet wavelength region due to the use of a reflection system as an element thereof and an exposure apparatus using the same.
2. Related Background Art
In a photolithography process for making semiconductor device or the like, there is used a projection exposure apparatus in which a pattern image of a photomask or reticle (collectively referred to as "reticle" in the following) is reduced to about 1/4 to 1/5 by way of a projection optical system and then thus reduced pattern image is projected on a wafer (or glass plate, for example) coated with a photoresist or the like. As the integration of the semiconductor devices and the like is improved, there has been a demand for a higher resolution in the projection optical system used in the projection exposure apparatus. In order to satisfy such a demand, there have been occurred necessities for shortening the wavelength of illumination light and increasing the numerical aperture (N.A.) of the projection optical system.
However, as the wavelength of the illumination light becomes shorter, a fewer kinds of glass materials can be practically used due to the light absorption. Specifically, when the wavelength of the illumination light is 300 nm or less, only synthetic quartz and fluorite can be listed as practically usable glass materials. Accordingly, when the projection optical system is constructed by a refraction system alone, it becomes difficult for various aberrations such as chromatic aberration to be corrected. On the other hand, when the projection optical system is constructed by a catoptric system alone, there is no chromatic aberration. Therefore, there has been proposed a technique in which a reduction projection optical system is constructed by a so-called catadioptric optical system which combines a catoptric system and a dioptric system together.
A catadioptric reduction optical system having a beam splitter for changing optical paths in order to input and output a luminous flux with respect to a catoptric system is disclosed, for example, in Japanese Unexamined Patent Publication Hei No. 2-66510, No. 4-235516, and No. 5-72478. All the concave mirrors included in the catadioptric reduction optical systems disclosed in the above-mentioned publications are convergent mirrors having a reductive magnification.